Materials to shield against electromagnetic fields are becoming increasingly important in industry. Electromagnetic waves can, for instance when used in high-frequency electronic components, lead to interference in nearby components. Furthermore, there is a general desire to reduce electromagnetic radiation.
In recent years, composites, in particular, have been found to be useful for these purposes. Such composites generally comprise a magnetically active material, usually a ferrite, in a polymer matrix. Thus, for example, the German patent application DE 102 22 459 A1 discloses such a composite comprising a modified ferrite powder, electrically conductive nonmetal particles and a magnetic binder in a polymer matrix. A preferred ferrite for this purpose is manganese-zinc ferrite.
Similar materials are now also commercially available and are marketed, for example, by EPCOS AG under the name C350 or C351 (“Ferrites and Accessories”; EPCOS AG; Edition 09/2006), by Vogt (product catalogue “Inductive Component and Modules”; Vogt Elelctronic Components GmbH; Edition 2005) and by Kerafol GmbH (safety data sheet “Keratherm Typ 86/XX”; 6 2000).
However, the known materials have a series of disadvantages. One important aspect is that the density of the known materials is generally below 2.5 g/cm3 because of a very high proportion of polymers. A considerable volume is therefore necessary for effective shielding.
A further disadvantage is that these known materials have to be processed by injection moulding and usually have a very low mechanical strength. Thus, the tensile strengths are generally below 2.5 N/mm2.
Ethylene-vinyl copolymer or polyamide is predominantly used for the polymer matrix in the known composites of this type. This limits the use temperature to regions significantly below 200° C., which leads to the materials not being able to be used for all purposes.
In respect of the magnetic properties, too, the materials were not yet fully satisfactory. Although it was possible to realize initial permeability values μi of from 7 to 20 by means of these materials, it was not possible to achieve values below this range nor to establish a targeted setting of the value in advance. The materials were also unsatisfactory in respect of the achievable saturation magnetization or the coercivity Hc.
A further composite and a production process for this are disclosed in the patent application EP 1 564 767 A2. This patent application proposes sieving a magnetic powder material in order to obtain two fractions. The one fraction which remains on the sieve is subsequently coated with an insulating and binding dielectric material. The fine fraction which passes through the sieve is subsequently mixed with the coated magnetic powder again and the mixture is pressed at a temperature below the glass transition temperature. The compressed product is heated and baked with a thermoplastic resin. The magnetic permeability μi attains values of about 350. The citation starts out from particle sizes in the range from 0.1 mm to 0.125 mm.
Finally, the European patent application EP 1 669 408 discloses a composite and a processing or production process for this material. This document states that processing by spray drying is possible for a suitable mixture of water, polymer and the magnetic powder, for example manganese-zinc ferrite. The mixing ratios of magnetic particles to polymer given in the examples lead to a density below 3 g/cm3. The ferrites used had particle sizes in the range from 1.6 to 8.5 μm. In this patent application, a resin having unsaturated vinyl units is used as polymer material. This leads to hard and relatively brittle particles being obtained after spray drying. These can then be used further in unprocessed form as powder, dissolved as paint or by means of casting techniques. However, dry pressing is ruled out because of the brittleness of the particles.
Furthermore, EP 1 790 460 A1 discloses an insulating coating for an electrical component which comprises a thermoset polymer and a filler material in the nano range.
DE 10 2004 041 649 A1 discloses a magnetorheological elastomer whose mechanical properties vary with the applied magnetic field, which comprises magnetizable particles having a bimodal size distribution.
US 2007/0102663 A1 discloses a magnetic paste having multimodal magnetic constituents.
US 2002/0190236 A1 discloses a magnetic granular material which is produced by means of spray drying and contains polymer constituents in addition to a magnetic constituent. The granular material is sintered at 1300° C., with the polymer vaporizing completely. A simple process is disclosed in EP 1 818 954 A1.